In the 1960s, many trials for the preparation of an organic electroluminescence device were reported using conjugated materials generally having fused aromatic rings (U.S. Pat. No. 3,172,862, issued 1965; U.S. Pat. No. 3,173,050, issued 1965). The efficiencies and lifetimes of these organic EL devices were much lower than those obtained from inorganic systems at the same time, so research mainly focused on the inorganic materials. The reason for the low luminance of the early organic EL devices is the highly resistive EL medium, which prevents the efficient injection of carriers into the light-emitting layer. Tang and VanSlyke solved this problem successfully in the late 1980s (Tang and VanSlyke, Appl. Phys. Lett. 1987, 51, 913). They improved the performance of an organic EL device significantly by using a structure with two thin layers: a hole-transporting layer of an organic substance laminated on an organic emitting layer. This work revived the research on organic EL devices, and resulted in the development of a new generation of light-emitting diodes with organic dyes. Since then, much work has been done to further improve the efficiency, stability, colour purity and so forth of such devices (U.S. Pat. Nos. 5,141,671; 4,539,507; 6,020,078; 5,935,720; 5,972,247; 5,593,788; 4,885,211; 5,059,862; 5,104,740; 5,069,975; 5,126,214; 5,389,444; 6,165,383; 6,245,449; Chen, Shi and Tang, Macromol. Symp., 1997, 125, 1; Segura, Acta. Polym., 1998, 49, 319; Mitschke and Bauerle, J. Mater. Chem. 2000, 10, 1471).
Performance stability of EL devices is a very important consideration for practical applications. A number of factors are known to influence the device stability. These include thermal and chemical stability of materials (including hole-transporting materials, electron-transporting materials and emitting materials, etc.), carrier mobility of electron and hole-transporting materials, the configuration of device, as well as environmental factors. To address these problems, several useful methods have been developed and are well documented. For example, one is to dope a strongly emitting material into a host material to form a guest-host system. It has been shown that an organic EL device with good efficiency and high stability, as well as desired colour with proper chromaticity, can be obtained by doping different strongly emitting materials into a host material such as tri-(8-hydroxyquinolinato)aluminum (AIQ3).
The thermally induced deformation of the hole-transporting materials is thought to be one of the main causes of degradation. Thus, many studies have been focused on the design and preparation on new hole-transporting materials, and the relationship between the properties (e.g. Tg) and molecular structure (U.S. Pats. No. 6,242,115; 6,333,521; EP 0,699,654 A1; Shirota, J. Mater. Chem., 2000, 10, 1). Isoindole derivatives have been synthesized by the reaction of phthalaldehyde and an amine in the presence of an alkylthiol which was used as the detection method for amino acids, peptides, and proteins. It has also been found that isoindole derivatives are highly fluorescent emitting in blue. Some highly fluorescent polymers containing isoindole moiety have been prepared and showed properties of high glass transition temperatures and high thermal stabilities. However, these materials have never been used in small molecule organic EL devices.
The object of the present invention is to provide novel compounds which are suitable for use as hole-transporting materials in organic electroluminescence devices. These compounds should be highly thermally stable, and should have a similar hole-transporting ability to known hole-transporting compounds.